Recent neuroradiological findings suggest that altered patterns of brain metabolism may precede cortically-mediated neuropsychological deficits in senile dementia of the Alzheimer's type (SDAT). These metabolic changes may be associated with patterns of attentional loss that are not detectable by standard neuropsychological tests. The principal aims of the proposed project are (a) to evaluate two systems of attention in early stages of SDAT in comparison to the normal elderly, and (b) to explore the contribution to early diagnosis of SDAT of assessment of patterns of attentional loss and altered brain metabolism. A unified neuropsychological model of attention in SDAT is proposed. The model links two aspects of attention, "attentional cost", and sustained attention capacity, with a corticocortical system and a subcortical thalamic projection system, respectively. Three interrelated experiments designed to evaluate these two attentional systems will be carried out. In Experiments 1 and 2 a probe-reaction time task will be used to evaluate the attentional cost of mental operations (e.g., stimulus encoding, response selection) in patients in the early stages of SDAT. The cost of processing will be examined as a function of processing type, cortical hemisphere subserving processing, and intensity of processing demands. In Experiment 3 sustained attentional capacity will be evaluated by a degraded letter-detection task. The results of these studies will provide valuable pilot data for carrying out more extensive investigations of the utility of assessment of attentional loss in early diagnosis of SDAT, particularly in combination with measures of brain metabolism. The attention data obtained from SDAT patients will be related to data on brain metabolic patterns being collected from the same patients at the Laboratory of Neurosciences. This unique neuroradiological/neuropsychological approach will provide valuable information relevant to early diagnosis of SDAT that would not otherwise be available at low cost. Findings from this study may also indicate that a subtle attentional loss in the early stages of the disease predicts subsequent rate of decline. Such a result would potentially help in the development of appropriate management and treatment modalities for patients with SDAT.